Dark matter

        Twenty years ago, scientists believed that the expansion of universe was due to momentum obtained during the Big bang. Scientists believed that due to gravity expansion will eventually slow down. According to the physics law the expansion of universe should slow down. But as years passes, the slowing was not observed.
             In 1998, when the Hubble space Telescope was launched which observed a very distant supernovae and were surprised that expansion of universe had not slowed due to gravity, instead it was accelerating.
         It is estimated that expansion rate of universe is about 72 kilometers per second per mega parsec and this is called "Hubble constant". We can determine the present rate of expansion by measuring the velocities at which other Galaxies are moving away from us, using the Doppler effect.
           The total mass-energy of the universe according to standard model of cosmology contains 4.9% of ordinary matter, 26.8% of dark matter and 68.3% of dark energy. Hence dark matter and dark energy together constitute 95% of the total mass-energy of universe.
                  Dark matter is a hypothetical matter which has not been observed yet directly. Dark matter is not in the form of dark clouds of normal matter, because matter is made of particles called as baryons. And we know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them. And also " dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with the matter.
              The name " Dark matter " , itself indicated that it doesn't emit or interact with electromagnetic radiation, such as light and is thus invisible to the electromagnetic spectrum. And some hypothesis suggest the dark matter contents exotic matter which doesn't react with the normal matter but can still exerts gravitational pull.